The present invention relates to a magnetic resonance imaging device, comprising at least a main magnet system for generating a steady magnetic field in a measuring space of the magnetic resonance imaging device, a gradient system for generating a magnetic gradient field in said measuring space, and an eddy current shield system.
The basic components of a magnetic resonance imaging (MRI) device are the main magnet system, the gradient system, the RF system and the signal processing system. The main magnet system is also often called cryostat. The main magnet system comprises a bore hole defining a measuring space and enabling the entry of an object to be analyzed by the MRI device. The main magnet system generates a strong uniform static field for polarization of nuclear spins in the object to be analyzed. The gradient system is designed to produce time-varying magnetic fields of controlled spatial non-uniformity. The gradient system is a crucial part of the MRI device because gradient fields are essential for signal localization. The RF system mainly consists of a transmitter coil and a receiver coil, wherein the transmitter coil is capable of generating a magnetic field for excitation of a spin system, and wherein the receiver coil converts a precessing magnetization into electrical signals. The signal processing system generates images on basis of the electrical signals.
Magnetic resonance imaging (MRI) devices known from prior art usually generate a relatively high acoustic noise level which has to be minimized. On the one hand, acoustic noise is caused by vibrations of the gradient system, and on the other hand acoustic noise is caused by vibrations of the main magnet system (cryostat).
The acoustic noise generated by the gradient system vibrations can effectively be reduced by means of a vacuum chamber. See for example U.S. Pat. Nos. 6,404,200 and 5,793,210.
In order to further reduce the acoustic noise of the MRI devices, the acoustic noise generated by the vibrating main magnet system needs to be reduced. The main magnet system vibrations are caused by three excitation mechanisms, firstly by a structural transmission of vibrations from the gradient system to the main magnet system through gradient coil mounts, secondly by a magnetic excitation of the main magnet system due to the varying magnetic gradient-fields causing eddy currents in the wall of the main magnet system, and thirdly by an acoustic excitation of the main magnet system. The third excitation mechanism is not dominant for most MRI devices.
The first excitation mechanism causing vibrations of the main magnet system can be reduced effectively by using a compliant support for the gradient coils of the gradient system. See for example EP-A-1 193 507.
From U.S. Pat. No. 6,326,788 it is known that the magnetic excitation of the main magnet system can effectively be reduced by means of an eddy current shield system mounted rigidly on the gradient system. However, it is difficult to reduce eddy currents in the flange of the main magnet system by means of an eddy current shield system mounted on the gradient system.
From U.S. Pat. No. 6,437,568 it is known that the magnetic excitation of the main magnet system can effectively be reduced by using a non-conducting main magnet system. This has however drawbacks with respect to a boil-off effect, because heat is generated inside the main magnet system as a result of the fact that the main magnet system is non-conducting.